The present invention generally relates to apparatuses for controlling oscillation outputs of magnetrons, and more particularly to an apparatus for controlling an oscillation output of a magnetron by stably supplying a-microwave power.
In the fabrication of semiconductor devices, etching processes and CVD processes are performed by a microwave plasma treatment using a magnetron. To control the microwave output of the magnetron, the microwave power should be stably supplied to the magnetron and also be finely controlled. In the fabrication of LSIs, the microwave plasma treatment should be performed at a high speed and with a high accuracy.
FIG. 1 shows an example of a conventional apparatus for controlling an oscillation output of a magnetron. Referring to FIG. 1, the apparatus generally comprises a transformer T1, diodes D1 and D2, and high-voltage capacitors C1 and C2. A magnetron 100 is connected to a heater power supply 110. A two-phase power supply voltage having a frequency of 50 Hz and an A.C. voltage of 200 V is applied to a primary side of the transformer T1. The voltage of a secondary side of the transformer T1 is made m times the voltage of the primary side thereof. The voltage on the secondary side is subjected to a full-wave rectification by the diodes D1 and D2. A microwave power that is output from the apparatus is determined by capacitances of the high voltage capacitors C1 and C2. Thus, when the frequency of the microwave power is 50 Hz, a current Ib applied to an anode of the magnetron 100 has a waveform shown in FIG. 2. In this case, the microwave power is 1500 W.
FIG. 3 shows another example of a conventional apparatus for controlling an oscillation output of a magnetron. Referring to FIG. 3, the apparatus generally comprises a transformer T2, a thyristor circuit 101, and a rectifying circuit 102. A magnetron 100 is connected to a heater power supply 110. A two-phase power supply voltage having a frequency of 50 Hz and an A.C. voltage of 200 V is applied to a primary side of the transformer T2. The voltage of a secondary side of the transformer T2 is controlled by a current control on the primary side. A microwave power that is output from the apparatus is supplied to the magnetron 100 through the rectifying circuit 102. Thus, when the frequency of the microwave power is 50 Hz, a current Ib applied to an anode of the magnetron 100 has a waveform shown in FIG. 4. In this case, the microwave power is 1500 W.
In the prior art shown in FIG. 1, the microwave output of the magnetron 100 is fixed. Thus, the magnetron 100 could not be made to make the so-called soft-start and soft-stop. In addition, depending on a plasma impedance (hereinafter simply referred to as an impedance) in a chamber, an excessively large load may be applied on the magnetron 100, resulting in an oscillation failure. When the load on the magnetron 100 is excessively large, the cathode and anode of the magnetron 100 become short-circuited. For this reason, the current that flows through the anode of the magnetron 100 increases from a normal value between 0.4 and 0.7 A to a large value of 200 A in a short time of several tens of .mu.sec. As a result, there was also a problem in that the magnetron 100 itself or the apparatus will be damaged.
On the other hand, in the prior art shown in FIG. 3, the microwave output of the magnetron 100 can be variably controlled. However, in this case, to obtain a high microwave output, the peak value of the current applied to the anode of the magnetron 100 must be set to a high value. As a result, the load on the magnetron 100 became high, resulting in shortening the serviceable life of the magnetron 100.
Therefore, in the prior arts described above, the microwave output of the magnetron 100 could not be stably controlled to stably generate the microwave plasma, and it was difficult to make the microwave plasma treatment at a high speed with a high accuracy.
On the other hand, an apparatus that generates plasma in the pulse form by use of pulse power has been proposed in a Japanese Laid-Open Patent Application No. 1-149965. In this proposed apparatus, a combination of the pulse width of a voltage or current supplied to the magnetron, the frequency, the amplitude, and the number of pulses is modulated, so as to control an intermittent timing in which the pulse voltage or current is not modulated. Thus, the pulse form of the generated plasma differs from the pulse form of the pulse voltage or current supplied to the magnetron. However, this proposed apparatus controls the combination of the pulse width of the voltage or current, the frequency, the amplitude, and the number of pulses, thereby requiring complex control and circuitry. As a result, there was a problem in that the apparatus became bulky and expensive.
Another apparatus that supplies a pulse voltage or current to a magnetron has been proposed in a Japanese Laid-Open Patent Application No.3-261136. In this proposed apparatus., the duty factor of the pulse voltage or current is set in the range of 1/2 to 1/50, and the pulse width (half width value of peak current) is set in the range of 0.1 to 20 .mu.sec. However, unless the duty factor and pulse width of the pulse voltage or current supplied to the magnetron are constant, there was a problem in that the serviceable life of the magnetron becomes short.
Furthermore, since each of the proposed apparatuses described above uses pulse modulation to control the microwave output of the magnetron, the intensity of plasma emission becomes irregular. When the intensity of plasma emission is irregular, there was a problem in that it is difficult to control a light-end-point detection unit that detects the intensity of plasma emission within a chamber in which the plasma is generated.